Many polymer solutions exhibit a cloud point phenomenon, which occurs at the cloud point temperature, above or below which the polymer is soluble and the solution is clear, but below or above which it becomes insoluble and precipitates and solutions become opaque. In most polymer-solvent systems solubility decreases with falling temperature, but in some cases involving polar polymers, the opposite occurs and the polymer suddenly precipitates at a specific, higher temperature, the cloud-point temperature, or lower critical solution temperature (LCST). If such clear to opaque transitions due to deswelling occur at a low enough temperature and are reversible, such polymer solutions or gels are useful in a variety of applications such as self-activating sunscreens or temperature dependent drug release systems. Polymer occurring at low temperatures have been described in Japanese patent Nos. 85 190444; 85 208336; and 86 66707; they consist of aqueous solutions or gels of poly-isopropylacrylamide and of isopropyl acrylamide/N-methylolacrylamide copolymers and of pyrrolidyl or piperidyl/acrylamide copolymers. Besides these acrylamides, N-iso-, N-n-, N-cyclopropylacrylamide and the corresponding methacrylamides are described in these patents, as well as N,N-diethylacrylamide as the only disubstituted acrylamide. Only these acrylamides or methacrylamides have a sufficiently hydrophobic component as part of their structure to form homopolymers, which can change from a hydrophilic water binding to a more hydrophobic water excluding structure at relatively low temperatures.
It has now unexpectedly been discovered that copolymers of N,N-dimethylacrylamide (DMA), which is very hydrophilic, but whose homopolymer by itself does not form a thermo-reversible aqueous solution, and hydrophobic C.sub.1 -C.sub.18 alkyl acrylates or alkoxy-alkyl acrylates, as well as aryl and aralkyl acrylates, can be used to make aqueous solutions or gels, which exhibit LCST's, temperature dependent, reversible cloud points and swelling changes at low temperatures.
This was especially unexpected, since crosslinked hydrogels formed by copolymerization of dimethylacrylamide, 20-80% by weight, with C.sub.1 -C.sub.4 alkyl acrylates or methacrylates, 80-20% by weight and a small amount of crosslinking agent are described in U.S. Pat. No. 4,067,839 as colorless clear gels and even especially claimed as contact lenses; indeed, it was very surprisingly found, that if within the range of the above specification only alkyl acrylates are used, the obtained hydrogels exhibit thermotropic, reversible cloud points between 0.degree. to 90.degree. C., making them very unsuitable as contact lenses, but useful as heat activated sunscreens for example.
The cloud-point and LCST phenomenon of the novel N,N-dimethylacrylamide copolymer gels and solutions was unexpected also in light of U.S. Pat. No. 4,625,009, wherein crosslinked copolymers of N,N-dimethylacrylamide with acrylic- or methacrylic acid and a wide variety of alkyl-acrylates and methacrylates are described as hydrogels with at least 60% water content, also for use as soft contact lenses; in all these hydrogels incorporation of acrylic- or methacrylic acid as a comonomer is absolutely essential in order to achieve the high degree of swelling in water and clarity. We have shown, that in the absence of the extremely hydrophilic, strongly hydrogen bonding comonomers acrylic or methacrylic acid, crosslinked compositions of U.S. Pat. No. 4,625,009 would not be as highly swollen as claimed and would not be clear at eye-temperature.